1. Field of the Invention
The present invention relates to a method for producing a semiconductor device including a thermal treatment step, and to a semiconductor device.
2. Background of the Related Art
The field of power semiconductor devices has witnessed, in recent years, active development and research of products, having already begun to be put into practical use, that utilize wide band gap semiconductors such as nitride-based semiconductors, for instance gallium nitride (GaN)-based semiconductors. As is known, wide band gap semiconductors are advantageous, as compared with conventionally used silicon (Si), in that the former allows producing high-breakdown voltage semiconductor devices with low on-resistance, and enable operation at high temperatures. By virtue of such advantages, nitride-based semiconductors are expected to replace Si-based materials, as materials of power devices such as inverters and converters.
A thermal treatment, i.e., activation annealing at a high temperature, for crystal recovery and/or impurity activation, has to be performed after ion implantation in the production process of a nitride-based semiconductor device that is produced using a nitride-based semiconductor. However, when the activation annealing of a nitride-based semiconductor such as a GaN-based semiconductor involves setting a heating temperature of 800° C. or higher, so-called nitrogen loss occurs in that nitrogen (N) in the composition escapes from the nitride-based semiconductor, and the latter starts decomposing.
Therefore, a method is resorted to that involves performing activation annealing after formation of a protective film (cap layer) comprising a material of high heat resistance, on the upper layer of a nitride-based semiconductor layer. Japanese Patent Application Publication No. H08-186332 (Patent Literature 1), Japanese Patent No. 2540791 (Patent literature 2), and J. C. Zolper et al., “Sputtered AlN encapsulant for high-temperature of GaN”, Appl. Phys. Lett. 69(4), 22 Jul. 1996 pp. 538-540 (Non-patent literature 1) disclose methods that involve performing a thermal treatment in nitrogen while protecting the surface using an AlN layer as a protective film.
Activation annealing after impurity doping, for instance by ion implantation, requires heating at a temperature that is about ⅔ of the melting point of the material that makes up the semiconductor layer. Specifically, a heating temperature ranging from about 1500° C. to 1700° C. is envisaged in a case where a nitride-based semiconductor such as GaN is used as the semiconductor material.
With all that said, it has been reported, for instance, in X. A. Cao et al., “Ultrahigh Si+ implant activation efficiency in GaN using a high-temperature rapid thermal process system”, APPLIED PHYSICS LETTERS 73 (1998) pp. 229-231, (Non-patent literature 2) and K. A. Jones et al., “The Properties of Annealed AlN Films Deposited by Pulsed Laser Deposition”, Journal of ELECTRONIC MATERIALS, Vol. 29, No. 3 2000 pp. 262-267 (Non-patent literature 3), that even when using an AlN layer as a protective film, however, pits may occur in the AlN layer, or the latter may decompose, at such high-temperature regions, so that, as a result, the AlN layer no longer function as a protective film. For instance, Non-patent literature 2 reports the occurrence of pits in an AlN layer due to heating at a temperature of 1400° C. or higher, as an example where heating is performed at a temperature up to 1500° C. as a high-temperature region. When pits occur in the AlN layer that is used as a protective film during the thermal treatment, the likelihood increases of the occurrence of release of nitrogen, that makes up the underlying nitride-based semiconductor layer, through the pits.
Such being the case, the temperature of activation annealing in the related art has been limited to about 1300° C. In a case where activation annealing is performed after impurity doping, for instance by ion implantation or the like, it is impossible to elicit sufficient impurity activation and crystallinity recovery, in a semiconductor layer, at a heating temperature of about 1300° C. The problem of, for instance, lowered carrier mobility in the semiconductor device that is produced arises as a result in related art. A further problem is that, in particular in a case where a p-type region is formed by ion implantation, it has not been possible to obtain a sufficient p-type carrier concentration for the amount of implanted impurity, due to the n-type carrier compensating effect elicited by defects.
In view of the above, it is an object of the present invention to provide a method for producing a semiconductor device and a semiconductor device that allow a high-temperature thermal treatment to be carried out stably and effectively, while preventing nitrogen loss from a nitride-based semiconductor layer that makes up a semiconductor device.